✔️ The graph shows the rod’s temperature (variable t) along its length; its endpoint temperatures are 50C and 80C (particular values t1 and t9).
✔️ The graph shows the rocket's speed (a single variable) over the past ten minutes. Comparing the rocket's speeds at two different instants (multiple values of the variable),...
❌ Comparing the rocket's speed,...
(A comparison necessarily regards multiple entities.)
The bar chart gives information about the percentages of the population living in urban areas in the world and in different continents.
Correction:
✔️ The bar chart gives information about the percentage of urban dwellers in the population (a single variable) of the world and of different continents.
✔️ The bar chart gives information about the urban-dwelling percentage of population (a single variable) in the world and on different continents.
This screenshot is phrased perfectly fine. Also, noting that “internet penetration rate”, “internet-using percentage of population” and “internet-using population percentage” are synonymous:
✔️ The graph shows three countries' internet penetration rate (a single variable) between 1999 and 2009. Comparing the penetration rates for Mexico in 1999 and 2009 (multiple values of the variable),...
❌ The graph shows three countries' internet-using percentages of population between 1999 and 2009. Comparing the population percentage for Mexico in 1999 and 2009,...
The line graph compares the percentage of people in three countries who used the Internet between 1999 and 2009.
We are comparing three countries’ trends. As the “percentage” being discussed is a single variable, it is indeed correct to be in the singular form. However, the quoted sentence's ambiguous phrasing can be improved:
✔️ The graph compares three countries' internet-using percentage of population between 1999 and 2009.
✔️✔️ The graph compares three countries' internet penetration rate between 1999 and 2009.
Finally, here's a contrived but technically correct sentence that, due to the plural form “percentages”, is ostensibly comparing multiple percentage figures, like USA 2001’s 40% and Mexico 2005’s 30%: “this graph provides a comparison of the percentages of the population who used the internet, in three countries, between 1999 and 2009.”
In a different sample essay, but very similar in question style, he wrote, “the participation rates of...”
✔️ The participation rates of Korea and Sweden are different.